Vacuum flask



FIG. 1 is a front elevational view of a vacuum flask showing my new design;

FIG. 2 is a rear elevational view thereof;

FIG. 3 is a top plan elevational view thereof;

FIG. 4 is a bottom plan view thereof;

FIG. 5 is a left side elevational view thereof;

FIG. 6 is a right side elevational view thereof;

FIG. 7 is a top perspective view thereof. 

The ornamental design for a vacuum flask, as shown and described. 